There is compelling evidence that carcinogenesis is a multistep process and multiple genetic lesions are necessary to develop cancer in human. Among the genetic lesions, the dysfunction of p53 protein (because of mutation of p53 gene or infection by "high risk" human papillomaviruses [HPV], e.g., type 16 or 18 HPV) is the most frequently found genetic disorder in human cancers including oral cancer. In spite of such frequent p53 dysfunction in oral cancer cells, altered p53 function (by transfection with HPV DNA or mutant p53 cDNA) alone is not sufficient for neoplastic conversion of normal human oral keratinocytes in vitro. Therefore, the dysfunction of p53 protein may be an early event and also be necessary for subsequent genetic disorders of other genes to convert normal cells to tumor cells. In fact, our recent studies show that human oral keratinocytes containing negligible amount of wild-type (wt) p53 protein (because of HPV transfection) convert to tumorigenic cells when exposed to tobacco-carcinogens, but the normal counterpart does not. Inasmuch as wild-type p53 protein plays a major role in the regulation of cell cycle arrest, we hypothesize that normal human oral keratinocytes expressing wild-type p 53 protein have the ability to establish transient delays in the progression cell cycle when exposed to tobacco-carcinogens, that will allow cells to repair damaged DNA caused by tobacco-carcinogens. However, oral keratinocytes with p53 dysfunction lose such ability and fail to repair damaged DNA, resulting propagation of genetic errors and eventual conversion to tumor cells by tobacco-carcinogens. To test this hypothesis, we will investigate the effect of tobacco-carcinogens on cell cycle, expression of cell-cycle regulating genes (i.e., p53, WAF 1/CIP1, gadd153, and gadd45) and mdm-2 (a gene whose expression is upregulated by wt p53 protein), genotoxicity (single strand DNA breaks), and unscheduled DNA synthesis (DNA repair) in normal human oral keratinocytes expressing wt p53 protein, human oral keratinocytes expressing mutant p53, and human oral keratinocytes containing negligible amount of wild-type p53 protein. These proposed studies would help us gain more insight into molecular mechanisms of tobacco-related oral carcinogenesis.